For years, it has been thought that humans have a constant metabolic energy rate.
It was assumed that you would require the same total energy to run one
mile, no matter if you ran it in 5 minutes or 10 minutes. Even though
your energy burn rate would be higher at faster speeds, you would get
there in half the time.

Turns out, however, that each person has an optimal running pace that
uses the least amount of oxygen to cover a given distance. The
findings, by Karen Steudel, a zoology professor at Wisconsin, and Cara
Wall-Scheffler of Seattle Pacific University, are detailed in latest
online edition of the Journal of Human Evolution.

Steudel's team tested both male and female runners at six different
speeds on a treadmill while measuring their oxygen intake and carbon
dioxide output. As expected, each runner had different levels of
fitness and oxygen use but there were ideal speeds for each runner that
required the least amount of energy.

Overall, the optimal speeds for the group were about 8.3 mph (about a 7:13 minutes per mile) for males and 6.5 mph (9:08
min/mile) for females.

The most interesting finding: At slower speeds, about 4.5 mph (13 min/mile), the metabolic efficiency was at its lowest. Steudel
explains that at this speed, halfway between a walk and a jog, the runner's gait can be awkward and unnatural.

"What that means is that there is an optimal speed that will get you there the cheapest," Steudel says.

So, why is a zoology professor studying running efficiency? Steudel's
previous work has tried to build a theory of why our early ancestors
evolved from moving on four limbs to two limbs, also known as
bipedalism. She has found that human walking is a more efficient method
of getting from point A to point B than on all fours. It might also
have been an advantage for hunting.

This latest research could offer some more clues of how we moved on
to running. Steudel explains, "This is a piece in the question of
whether walking or running was more important in the evolution of the
body form of the genus Homo."